1. Field of the Invention
The present invention relates generally to an optical communication system and an optical amplifier suitable for long-haul and large-capacity transmission, and more particularly to an optical communication system applicable to WDM (wavelength-division multiplexing) and an optical amplifier suitable for WDM.
2. Description of the Related Art
In recent years, research and development on application of an optical amplifier to an optical communication system have been intensively pursued. For example, importance of a booster amplifier, optical repeater, and preamplifier having an EDFA (erbium doped fiber amplifier) has become apparent.
Conventionally known is an optical amplifier comprising an optical amplifying medium for amplifying signal light and means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including the wavelength of the signal light. In the case that the optical amplifying medium is an EDF (erbium doped fiber) having a first end and a second end, the pumping means includes a pump light source for outputting pump light having a proper wavelength, and means for supplying the pump light into the doped fiber from at least one of the first end and the second end. In the case that the optical amplifying medium is provided by a semiconductor chip, the pumping means includes means for injecting a current into the chip.
To greatly increase a transmission capacity, a WDM system (wavelength-division multiplexing system) has been proposed. The WDM system includes a first terminal station for outputting WDM signal light (wavelength-division multiplexed signal light) obtained by wavelength-division multiplexing a plurality of optical signals having different wavelengths, an optical transmission line for transmitting the WDM signal light output from the first terminal station, and a second terminal station for receiving the WDM signal light transmitted through the optical transmission line. To increase a transmission distance in the WDM system, one or more optical repeaters each having an optical amplifier are provided in the optical transmission line.
In applying the optical amplifier to the WDM system, gain tilt occurring in the optical amplifier must be considered. The gain tilt is based on the wavelength dependence of gain. In an EDFA, for example, the gain tilt changes with a change in total input power because of characteristics of homogenous broadening of an EDF. Accordingly, in operating the WDM system or the optical repeater, it is desirable to grasp the gain tilt of the optical amplifier and maintain a constant gain tilt.
In the optical amplifier or the optical repeater, a feedback loop for ALC (automatic level control) is usually adopted, so as to maintain the output level constant. In applying the optical amplifier adopting ALC to the WDM system, a target level in ALC for maintaining output power per channel constant changes with a change in the number of channels of WDM signal light. Accordingly, the conventional optical communication system cannot easily respond to a change in the number of channels.
It is therefore an object of the present invention to provide an optical communication system which can easily respond to a change in the number of channels in WDM.
It is another object of the present invention to provide an optical amplifier which can maintain a constant gain tilt.
It is a further object of the present invention to provide an optical amplifier which can maintain a constant gain tilt and allows automatic level control.
An optical communication system to which the present invention is applicable includes first and second terminal stations, an optical transmission line connecting the first and second terminal stations, and an optical repeater provided in the optical transmission line. The first terminal station includes a plurality of optical transmitters for outputting optical signals having different wavelengths, and a means for wavelength-division multiplexing the optical signals to output WDM signal light. The WDM signal light is transmitted by the optical transmission line, and received by the second terminal station. The optical repeater includes an optical amplifier for amplifying the WDM signal light.
In accordance with a first aspect of the present invention, the first terminal station further includes a means for detecting the number of channels of the WDM signal light, and a means for transmitting supervisory information indicating the number of channels to the optical repeater. The optical repeater further includes a means for detecting an output level of the optical amplifier, and a means for controlling the optical amplifier so that the output level detected becomes a target level. In the optical repeater, for example, the target level is set according to the supervisory information transmitted from the first terminal station.
In accordance with a second aspect of the present invention, the optical repeater further includes a means for detecting an output level of the optical amplifier, and a means for controlling the optical amplifier so that the output level detected becomes a target level. The target level is constant irrespective of the number of the optical transmitters being operated. Preferably, the optical signals to be output from some of the plurality of optical transmitters being operated are modulated by main signals, and the optical signals to be output from the other optical transmitters not being operated are continuous waves.
In accordance with a third aspect of the present invention, the optical amplifier includes an optical amplifying medium, a means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including the wavelengths of the WDM signal light, a light source for outputting compensation light having a wavelength included in the amplification band but different from the wavelengths of the WDM signal light, and a means for supplying the WDM signal light and the compensation light to the optical amplifying medium. The optical repeater further includes a means for detecting an output level of the optical amplifier, a means for controlling the optical amplifier so that the output level detected becomes a target level, and a means for controlling power of the compensation light so that the target level becomes constant irrespective of the number of channels of the WDM signal light.
In accordance with a fourth aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying medium having a first end and a second end, the first end receiving signal light; a first means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including a wavelength of the signal light; a second means operatively connected to the first end of the optical amplifying medium, for monitoring spectral characteristics of amplified spontaneous emission propagating in a direction opposite to a propagation direction of the signal light in the optical amplifying medium; and a third means for controlling a gain in the amplification band so that the spectral characteristics are maintained.
In accordance with a fifth aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying medium having an optical waveguide structure into which signal light is supplied; a means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including a wavelength of the signal light; a means for extracting spontaneous emission leaked sideways from the optical waveguide structure; a means for monitoring spectral characteristics of the spontaneous emission; and a means for controlling a gain in the amplification band so that the spectral characteristics are maintained.
In accordance with a sixth aspect of the present invention, there is provided an optical amplifier comprising first and second optical amplifier units and a means for cascading the first and second optical amplifier units. Each of the first and second optical amplifier units has the configuration of the optical amplifier in accordance with the fourth aspect of the present invention. This optical amplifier further comprises an optical attenuator having a variable attenuation factor, for attenuating amplified signal light output from the first optical amplifier unit; a means for branching amplified signal light output from the second optical amplifier unit into first branch light and second branch light; a photodetector for receiving the first branch light; and a means for controlling the attenuation factor of the optical attenuator so that an output level of the photodetector becomes constant.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.